1. Field of the Invention
The present invention relates to electrical apparatus for measuring AC electrical energy quantities, and more specifically, to electrical apparatus implementing indicator diagram principles.
2. Description of the Prior Art
The electromechanical rotating disc type of watthour meter continues to enjoy almost exclusive use in electrical metering applications. However, other apparatus are being sought which will provide additional services, such as load control and time of day metering, on a cost competitive basis. This search has lead to the development of a multitude of apparatus capable of implementing a wide variety of techniques for determining AC electrical energy quantities.
One such technique for determining the value of an electrical energy quantity to be measured is a system based on a conventional analog multiplier circuit for computing the product of the current and voltage components. Such a system is disclosed in U.S. Pat. No. 3,864,631 and is assigned to the assignee of this invention.
A second technique, that is described in U.S. Pat. No. 3,764,908 and is assigned to the assignee of this invention, is for a semiconductor device having a logarithmic computing characteristic. Voltage and current related signals are input to the device which produces an output signal that is equal to the product of the input signals.
Another technique, known as time division multiplication, is illustrated by U.S. Pat. No. 3,794,917. Briefly, a signal proportional to the voltage on an electrical distribution system is sampled to derive a pulse width modulated output signal having a pulse duration proportional to the amplitude of the voltage signal. This signal controls a gating network which samples a current input signal at a rate proportional to the amplitude of the voltage signal. The resultant output signal is the product of the voltage and current and consists of a series of pulses having an amplitude proportional to the magnitude of the instantaneous current and a pulse width proportional to the magnitude of the voltage. After the product of the voltage and current signals is thus obtained, the resulting output signal is proportional to the average power consumed by the load. The output signal is then applied to a voltage to frequency converter which provides a pulse train wherein each pulse is proportional to the total energy consumed. The pulse train is used to drive mechanical counters, magnetic recorders, or electrical counting circuits and thereby provides an indication of the total electrical power consumed by a load.
It has been found that the analog electronic multiplying techniques are sometimes difficult to apply in order to obtain the desired accuracies. Accurate, driftfree analog multipliers and square root computing circuits are often expensive and difficult to obtain. Also, analog integration circuits required in the analog electronic power measuring apparatus produce undesired drift and variations over long time intervals. In time division multiplication circuits, it is known that frequency dependent sampling occurs with the associated digital integration also having a similar dependency upon variations of the integrating capacitors.
A further technique, digital processing, is illustrated by U.S. Pat. No. 4,077,061 and is assigned to the assignee of this invention. In this technique, the voltage and current signals are sampled at random intervals and converted into digital form. The digital signals are then processed by a digital computer which computes the instantaneous values of electrical energy quantities and accumulates the instantaneous values of the computed quantity over a predetermined time interval to provide a measurement of various electrical energy quantities.
Ideal digital measuring apparatus utilize analog to digital converters having very high speeds and high resolutions. However, these two characteristics are competing from a design standpoint since more time is needed in order to achieve the higher resolutions. The higher resolution results in binary words of greater lengths which require associated circuitry that is more complex and expensive. Further, complex digital processing circuits capable of operating at high speeds require larger and more expensive power supplies. Slower sampling speeds permit digital processing at lower speeds to simplify the digital processing circuitry. A reduction in resolution permits handling of shorter binary words for producing the digitized binary representations of the analog signal amplitudes. However, the reduced sampling rates and lower digitizing resolution have a corresponding reduction in the accuracy of the digital representations of the sample amplitudes and a reduction in the true digital representation of each complete cycle of the analog input signals.
A still further technique for determining the value of an electrical energy quantity to be measured is a system which determines the area of an indicator diagram. The area enclosed by an indicator diagram is proportional to the instantaneous power per cycle delivered to a load. The system disclosed in U.S. patent application Ser. No. 919,874 filed on June 26, 1978 and assigned to the assignee of this invention is an example of the indicator diagram technique. The area enclosed by the indicator diagram is determined by sampling a signal related to the current component whenever unit changes occur in the magnitude of the time integral of a signal related to the voltage component. This provides successive sample values of the signal related to the current component which are accumulated to provide an indication of the area enclosed by the indicator diagram. Further accumulation provides an indication of the energy consumed.
The present invention is an apparatus for implementing this last technique. More specifically, it is a system for computing the area enclosed by an indicator diagram.